The present invention relates to a continuous coagulation process of polytetrafluoroethylene (PTFE) prepared by a polymerization process in dispersion or emulsion, to obtain wet fine powders of non thermoprocessable PTFE (or modified PTFE).
More specifically, the invention relates to a continuous coagulation process of PTFE fine powders, comprising the aggregation/gelification of the PTFE latex by a capillary tube, and subsequent granulation (coagulation) by mechanical stirring.
Non thermoprocessable fine powders of PTFE or modified PTFE obtained by the process of the present invention are characterized by a high apparent density, xe2x89xa7470 g/l, an average diameter (D50) higher than 200 micron, preferably from 400 to 600 micron, and characterized by a narrow distribution of the particle diameters, said distribution, defined as a ratio between the weight of the particles having a diameter in the range 0.7-1.3 times the average diameter of the particles and the total weight of the polymer particles, being higher than 50%, preferably higher than or equal to 60%.
It is known in the prior art that PTFE fine powders are obtained by the polymerization in dispersion. In this process a sufficiently high surfactant amount is used so as to be able to stabilize the PTFE colloidal particles and a soft stirring is applied during the polymerization to avoid the polymer coagulation (precipitation). Subsequently the latex obtained from said process is coagulated and the powder obtained from the coagulation is called xe2x80x9cfine powderxe2x80x9d. The conventional coagulation process comprises the following steps:
latex dilution with water, optionally addition of a destabilizing electrolyte,
latex mechanical stirring to obtain the formation of aggregates/gels,
granulation (coagulation) of the aggregates/gels by mechanical stirring,
PTFE flotation from water,
mechanical separation of the wet fine powder from the coagulum water.
Said PTFE fine powders are transformed by the lubricated extrusion process to obtain the manufactured articles.
The lubricated extrusion process is much more productive and easily automatizable the more the fine powders are flowing and have a high apparent density. Besides, the average particle diameter is preferably higher than 400 micron and the particle diameter distribution as above defined should be at least higher than 50%, preferably higher than or equal to 60% to obtain an improved flow.
Various processes to obtain fine powders, both in batch and in a continuous way, are known in the prior art. With the batch processes apparent densities of about 400 g/liter (see comparative Examples) or slightly higher are obtained and the particle average diameter (D50) is in the desired range. However the fine powders show the drawback to have a particle diameter distribution as above defined lower than 50%, and therefore unsuitable. Another drawback of the batch processes is that from the industrial point of view they are more expensive than the continuous processes. It is known to the skilled that generally it is not possible to transform the batch processes to obtain PTFE fine powder in continuous processes.
In U.S. Pat. No. 3,046,263 a continuous coagulation process of the PTFE latexes is described, comprising the following steps:
phase of strong mechanical stirring of the polymerization latexes using a specific power of 1-100 CVxc3x97sec/gallon (196 kJ/m3-19,600 kJ/m3), preferably using a centrifugal pump with an average time of the latex residence in the pump of 2 seconds;
aggregate or gel formation by passing through a capillary tube having hydraulic resistance of 0.5-20 p.s.i. (xcx9c3.4-136 kPa);
granulation in the presence of air by mechanical stirring with specific power of 0.25-50 CVxc3x97sec/gallon (49 kJ/m3-9,800 kJ/m3);
subsequent separation of the PTFE fine powder from water.
The drawback of said process is that the capillary tube is easily obstructed whereby the process must be often interrupted to clean the capillary tube. Furthermore it has been found by the Applicant that also in the first phase of the latex preparation which must be introduced in the capillary tube for gelification, the partial formation of PTFE granules takes place causing the obstruction of the centrifugal pump. This is a further drawback of the above process.
Another continuous process which avoids the drawbacks of the process described in the previous patent is reported in U.S. Pat. No. 5,977,295. In said process a capillary tube is not used, since considered unsuitable for a continuous process due to the above drawbacks. The PTFE latex deriving from the polymerization in emulsion is fed to a high shear apparatus having rotating elements to obtain aggregates/gels (gel phase). The so obtained gel phase is fed to the coagulation phase in a column by applying shear forces to the gel phase. According to U.S. Pat. No. 5,977,295 by this process high apparent densities, a good average particle diameter are obtained, but the particle diameter distribution is not mentioned. The drawback of said process resides in that a very complicated and expensive high shear apparatus Is used, which requires rotor peripherical rates of the order of 2 to 30 m/s and free space between stator and rotor of about 0.5 mm. This requires the water fluxing on the rotating parts for the correct working of the shear apparatus and of the process described therein. Besides, even though a so complicated apparatus is used, the shear is not perfectly uniform whereby it is not possible to obtain a narrow distribution of the particle diameters. This leads to the above drawbacks.
The need was felt to have available a continuous process for the preparation of wet PTFE fine powders, overcoming the drawbacks of the prior art and not requiring the use of complicated and expensive apparatus to obtain high shears to reduce the unhomogeneity of the obtained fine powders, and allowing to obtain PTFE fine powders having an improved flow and high apparent density and narrow distribution of the particle diameters, as above defined, higher than 50%, preferably higher than or equal to 60%.
An object of the present invention is a continuous coagulation process of PTFE or modified PTFE fine powders, comprising:
a1) dilution in a lift of a PTFE or modified PTFE latex obtained from the polymerization in dispersion (emulsion) up to a concentration from 5 to 25% w/w of PTFE, preferably from 8 to 20% w/w of PTFE or modified PTFE; and optional filtration of the obtained diluted latex,
b1) latex pressurization in the lift by an inert gas, preferably air, until a relative pressure, referred to the atmospheric pressure, in the range 3-40 kg/cm2 (0.3-4 MPa), preferably from 5 to 20 kg/cm2 (0.5-2 MPa), and still more preferably from 7 to 15 kg/cm2 (0.7-1.5 MPa),
c1) addition of an acid electrolyte solution, preferably nitric acid, to the PTFE or modified PTFE latex, in a in-line mixer, so that the pH is from 1 to 4, preferably from 1.5 to 3; the latex concentration and the pH being such as not to cause the formation of aggregates/gels in the in-line mixer,
d1) latex flowing from the mixer through a capillary tube under turbulent flow conditions, having a Reynolds number higher than 3,000, preferably higher than 5,000,
e1) granulation (coagulation) of the gel obtained in step d1) by mechanical stirring with a specific power from 1.5 to 10 kW/m3, maintaining the stirring until fine powder flotation,
f1) separation of the underlying water from the wet fine powder.
In the step d1) to obtain turbulent flow conditions of the latex, one preferably works as follows:
the capillary tube has an internal diameter such that the total capillary tube hydraulic resistance under the process conditions causes a pressure fall between the capillary tube ends from 3 to 40 kg/cm2 (0.3-4 MPa), preferably from 5 to 20 kg/cm2 (0.5-2 MPa), and still more preferably from 7 to 15 kg/cm2 (0.7-1.5 MPa),
the capillary tube length is from 0.1 to 30 m, preferably from 0.3 to 15 m, and still more preferably from 1 to 10 m,
the latex/gels rate inside the capillary tube is in the range 2-15 m/sec,
the capillary tube diameter is generally from 2 to 20 mm, preferably from 3 to 10 mm.
In a preferred embodiment the capillary tube length ranges from 0.1 to 3 m, still better from 0.2 to 1 m.
The combination of the capillary tube diameter, the latex concentration, the electrolyte pH and the latex linear rate through the capillary tube, must be such as to guarantee a turbulent flow of the latex inside the capillary tube.
In step d1) aggregation/gelification takes place. The capillary tube length as above defined is such to guarantee a complete aggregation/gelification of the latex and the absence of formation of the fine powder granules inside the capillary tube.
In FIG. 1 it is reported a preferred embodiment of the apparatus comprising the capillary tube for obtaining, starting from the latex, a polymer gel. The numbers have the following meaning:
1 Inlet for the latex.
2 Mixing chamber.
3 Pipe for electrolyte feeding.
4 Convergent section of 2.
5 Capillary tube.
6 Divergent section.
7 Outlet for the gel.
The latex entering from 1 is mixed with the electrolyte coming from 3 in the mixing chamber 2. Then the latex mixed with the electrolyte in chamber 2, enters in section 4 connecting the mixing chamber 2 with the capillary tube 5. The angle of the profile of the convergent section 4 can be from about 5xc2x0 to 15xc2x0, preferably of 10xc2x0. Then the latex mixture enters in the capillary tube 5, wherein gel formation takes place. The gel so formed passes to section 6. The angle of the profile of this section is not particularly limited, and in general ranges from 10xc2x0 to 20xc2x0. The gel at the end of 6, through the outlet 7, goes into the granulation apparatus (step e1) of the process of the invention).
By aggregates/gels it is meant that the polymer particles are dipped in the liquid phase and linked by crossed bonds so to form a thick network. The aggregates/gels properties significantly depend on the interactions of said two components (polymer and liquid). The liquid prevents the polymer network from being transformed into a compact mass and the polymer network prevents the liquid from coming out from the aggregates/gels. Depending on the chemical composition and other factors the consistence of the aggregates/gels varies from fluids to rather stiff solids.
At the end of step b1), while the previously pressurized latex is discharged in the mixer of step c1), a second lift is used which is fed with latex to be diluted according to step a1). When the first lift has been emptied, the latex fed into the second lift is at the end of step b1), and therefore the latex is fed again in the first lift.
For step e1) one works for example with the columns described in the prior art to have a continuous process, for example vessels having a heigth/diameter ratio higher than 2, equipped with stirrer (see the processes described in the above patents).
The PTFE or modified PTFE fine powders obtained from the process of the present invention are particularly suitable to the transformation by lubricated extrusion to obtain manufactured articles.
By PTFE according to the present invention the TFE homopolymer is meant, by modified PTFE, TFE copolymers with one or more comonomers are meant, having at least one unsaturation of ethylene type in an amount from 0 to 3% by moles, preferably from 0.01 to 1% by moles.
The comonomers which can be used are of both hydrogenated and fluorinated type.
Among the hydrogenated comonomers, ethylene, propylene, acrylic monomers, for example methyl methacrylate, (meth)acrylic acid, butylacrylate, hydroxyethylhexylacrylate, styrene monomers, such for example styrene can be mentioned.
Among the fluorinated comonomers it can be mentioned:
C3-C8 perfluoroolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
(per)fluoroalkylvinylethers (PAVE) CF2xe2x95x90CFORf, wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
(per)fluorooxyalkylvinylethers CF2xe2x95x90CFOX, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluorooxyalkyl having one or more ether groups, for example, perfluoro-2-propoxy-propyl;
fluorodioxoles, preferably perfluorodioxoles;
non conjugated dienes of the type:
CF2xe2x95x90CFOCF2CF2CFxe2x95x90CF2,
xe2x80x83CFX1xe2x95x90CX2OCX3X4OCX2xe2x95x90CX1F wherein X1 and X2, equal to or different from each other, are F, Cl or H; X3 and X4, equal to or different from each other, are F or CF3, which during the polymerization cyclopolymerize;
fluorovinylethers (MOVE) of general formula:
CFXAIxe2x95x90CXAIOCF2ORAI (A-I) wherein RAI is a C2-C6 linear, branched or C5-C6 cyclic (per)fluoroalkyl group, or a C2-C6 linear, branched (per)fluorooxyalkyl group, containing from one to three oxygen atoms; when RAI is a fluoroalkyl or a fluorooxyalkyl group as above it can contain from 1 to 2 atoms, equal or different, selected from the following: H, Cl, Br, I; XAIxe2x95x90F, H; the compounds of general formula:
CFXAIxe2x95x90CXAIOCF2OCF2CF2YAI (A-II), wherein YAIxe2x95x90F, OCF3; XAI as above are preferred; in particular (MOVE I) CF2xe2x95x90CFOCF2OCF2CF3 (A-III) and (MOVE II) CF2xe2x95x90CFOCF2OCF2CF2OCF3 (A-IV) are preferred.
The continuous process of the present invention, as said, allows to obtain PTFE fine powders having a narrow distribution (as above) of the particle diameter, whereby the fine powders show an improved flow. This property is combined with a high apparent density and with an average diameter preferably higher than 400 micron.
The continuous process of the invention leads to fine powders with reproducible properties. In the capillary tube used in the process according to the present invention, obstructions do not occur, even after long working periods. Besides from the industrial point of view the process does not require very expensive apparatuses having a high shear as above indicated.
The following Examples illustrate the invention but without limiting the scope thereof.